Heme b is the iron containing prosthetic group to an important pool of iron proteins known as the hemoproteins. Hemoproteins are functionally diverse, playing key roles in photosynthetic and respiratory electron transfer (e.g. cytochrome b6f, photosystem II, cytochrome bc1) among other fundamental biological processes. Heme b is the most naturally abundant heme structure, but data regarding hemes in the marine environment are limited. An investigation has been conducted to improve our understanding of heme b abundance in marine organisms through laboratory monoculture studies of three marine cyanobacteria grown under varying total iron concentration. The unicellular cyanobacteria Synechococcus sp. WH7803 was examined under three total iron concentrations: 12 nmol L-1 (low), 120 nmol L-1 (medium) and 1200 nmol L-1 (high). The marine diazotrophs (i.e. nitrogen fixers) Crocosphaera watsonii (WH8501) and Trichodesmium erythraeum (IMS101) were studied under six total iron concentrations between 0 and 120 nmol L-1. Cultures were analysed for heme b, chlorophyll a, particulate organic carbon (POC) and particulate organic nitrogen (PON) concentration. Nitrogen fixation rates and biophysical measurements (Fv/Fm and σPSII) were also obtained for diazotroph cultures. Field data regarding the concentration of heme b, chlorophyll a, POC and PON as well as nitrogen fixation rates were collected during two research cruises in the subtropical North Atlantic (STNA, D346) and tropical North Atlantic (TNA, D361); an oceanographic region known to demonstrate high nitrogen fixation rates and receive significant dust (iron) deposition from the Saharan and Sahel deserts of Western Africa. Cultures of Synechococcus sp. WH7803 showed evidence of iron stress at low iron treatments via reduced maximum growth rates (μmax), total biovolume and chlorophyll a concentration. This was also reflected by a significant reduction in cellular heme b content per unit carbon (heme:C) at the lowest iron concentration. An estimated heme b requirement between 1.0 and 1.5 μmol mol-1 C is proposed for Synechococcus sp. WH7803 in order to facilitate μmax. Chlorophyll a to heme b ratios (chl:heme) were significantly decreased in low iron cultures of Synechococcus sp. WH7803, suggesting b-type hemoproteins were conserved when iron stressed. Cultures of Crocosphaera and Trichodesmium were similarly influenced by the availability of iron, with reduced total biovolume and chlorophyll a concentration reported at low iron treatments. However, heme:C ratios were maintained at approximately 1.5 and 0.5 μmol mol-1 C for Crocosphaera and Trichodesmium cultures, respectively. A high iron requirement is associated with marine diazotrophs relating to the iron-rich non-heme nitrogenase enzyme complex responsible for nitrogen fixation. Nitrogen fixation rates increased as total iron concentration increased, with Trichodesmium demonstrating four-fold higher rates than Crocosphaera at corresponding iron concentrations. It has been suggested that relatively low heme b contents of Trichodesmium cultures resulted from increased nitrogen fixation activity. Furthermore, heme:C ratios of Crocosphaera and Trichodesmium were typically lower than five eukaryotic phytoplankton previously investigated, potentially related to the allocation of iron for nitrogen fixation. Mean heme:C ratios from cruises in the STNA and TNA were 0.64 and 0.66 μmol mol-1 C, respectively. Results could imply the region was iron stressed and/or dominated by cyanobacteria. Evidence is also presented suggesting a possible inverse relationship between nitrogen fixation and heme:C ratio in the TNA which could be attributed to natural populations of Trichodesmium.